1. Field of the Invention
The present invention relates to foams, articles containing foams, and methods for making those foams and articles, and more particularly to foams that are skinless and to the method of making said skinless foams.
2. Background of Related Art
Foams and articles containing foams can be prepared from many different chemical classes of materials and by many different processes. Organic polymeric film forming materials can be made into foams by processes that inherently liberate gases as a by-product of the polymerization reaction. Such polymeric materials can also be converted into foams by the addition of blowing agents (gas liberating agents) or gas injection into the reaction mass or softened states (molten, solution, etc.) of the polymer. The expanding gases form voids, cells, vesicles, or bubbles within the polymer mass.
Polyurethane polymeric materials such as those formed from organic polyisocyanates and polyhydric alcohols or polyols are a particularly desirable polymeric system for the production of foam materials. U.S. Pat. No. 3,808,162 describes polymers formed by reacting at least one polyol and at least one polyisocyanate in the presence of a particular catalyst system. Suitable polyisocyanates can be represented by the formula EQU R(NCO).sub.n
wherein R is a polyvalent organic group, for example, aliphatic, cycloaliphatic, aromatic, heterocyclic, heteroaliphatic, and/or combination thereof, and n is an integer of 2 to 5.
The formula EQU R.sup.1 (OH).sub.m
represents suitable polyols wherein R.sup.1 is one or more polyvalent organic groups selected from low molecular weight aliphatic groups and polymeric groups having an average molecular weight from 14 to 8000 and m is an integer of 2 to 5.
Characteristics of polyurethane, such as hardness and elasticity, can be controlled within relatively close limits by controlling the amount of crosslinking. The inclusion of trifunctional or higher polyfunctional components into the reaction mixture in predetermined amounts, or by building such further functionality into the isocyanate or the polyol reactants of the system to provide a functionality greater than two can produce a cross-linked elastomer. Thus, a small amount of a triol or other polyol such as 1,2,6-hexanetriol, pentaerythritol, trimethylol propane, glycerol, or polymeric compounds having more than two hydroxyls per molecule may be used. In addition to or in place of a polyol, the polyfunctional component can be a small amount of a triisocyanate or a polyisocyanate of greater functionality, such as that provided by the reaction of tolylene diisocyanate with trimethylol propane as mentioned above or with any of the aforementioned polyols. Typically, the amount of trifunctional component used is in the range of 1 to 10%, depending on the hardness desired and the average molecular weight of the crosslinking component used. Generally, the lower the equivalent weight and the greater the amount of the crosslinking component used, the harder is the polyurethane obtained.
The properties of the polyurethane foam may be greatly varied by selecting the backbone component, e.g., the polyol, for its possession of particular properties. The backbone or core component of the polymer may be selected for its flexibility, rigidity, hydrophilicity, oleophilicity (hydrophobicity), thermal stability, solvent resistance, etc. Some degree of backbone component properties can be carried into the polyurethane. U.S. Pat. Nos. 3,903,232; 4,137,200; 4,377,645; 4,384,050; and 4,384,051 describe examples of foams that have properties based in part upon the selection of the backbone component and the polyisocyanate.
A skin is formed on the exposed surface of polyurethane foam during conventional foaming processes.
Beneficially, the skin can prevent ready penetration of materials, such as water into the body of the foam. For example, water flotation devices are fabricated from polyurethane since the skin prevents penetration of water into the cells of the foam. The skin of a polyurethane foam is a distinct area, a continuous film that gives an impression of a smooth tough surface. The cross section of conventionally processed foams are typically characterized by an abrupt change from a high density skin to a cellular, less dense core. Although this outer surface may be interrupted by pores and craters, the average density of the skin is substantially greater than the density of the inner cellular core. The film component of the surface layer of skin of the foam usually comprises at least 25% of the total surface area, more usually more than 50% or 75% of the total surface, and can easily comprise more than 90% up to 100% of the surface. Polyurethane Handbook Chap. 7 (sec 7.1) (G. Oertel 1985).
However, the same skin that prevents water penetration is undesirable, when using the polyurethane foam for its absorbing properties. To beneficially utilize the absorbing properties, that is, to access the body of the foam, the skin component of the foam needs to be removed. "Skiving" is a method that results in the physical removal of the skin layer by cutting the surface away from the core. Removal of the skin from the foam necessitates additional process operations, while contributing to costs and raw material wastes. However, skiving does leave an effectively planar, two dimensional surface where the skin has been removed.